Mass Transport In Plants

Question 1

What’s the role of xylem ?

Answer 1

Transport water from roots and stem to leaves to replenish water lost from transpiration

Question 2

What’s transpiration ?

Answer 2

Water loss from leaves via evaporation

Question 3

What’s the role of phloem ?

Answer 3

Transport sugars and organic substances from leaves to rest of plant (translocation)

Question 4

What’re the structure and adaptions of xylem ?

Answer 4

No cytoplasm or organelles - no obstruction to flow of water

No end walls - allow continuous columns of water to form

Lignin - strengthen and Waterproof vessels. Prevent vessels collapsing under tension

Lateral pulls in cell wall - allow lateral movement around blockages

Question 5

What’s the cohesion tension theory ?

Answer 5

1. Water lost in leaf (transpiration)
2. Means lower W.P in leaf
3. Water moves from xylem to leaf down W.P gradient
4. Water pulled up xylem due to cohesion between water molecules which are held togtehr via hydrogen bonds
5. Forms continuous columns of water
6. Water adheres to lignin causing xylem to narrow
7. Increasing water pressure in xylem

Question 6

What are the factors that affect transpiration ?

Answer 6

Increase light intensity increases rate of it due to opening of stomata

Increase temp increases rate of it due to increase in water evaporation cause of high kinetic energy

Increase humidity difference between outside of leaf and in leaf increase the rate of it

Increase wind velocity increases the rate of it

Question 7

What’re the structure and adaptations of phloem ?

Answer 7

• Transport organic solutes from source cells to sink cells
• Sieve tube elements and companion cells form phloem tissue
• Sieve tube elements have no nucleus and few organelles they’re connected to each other by sieve plates
• Each sieve tube element has companion cells that carry out its living function for sieve cells
• companion cells have many mitochondria to produce ATP for transport

Question 8

What’s the mass flow hypothesis ?

Answer 8

1. Sucrose actively transported into sieve tube by companion cells
2. Lowers W.P in sieve tubes and water enters by osmosis from xylem
3. Produces higher hydrostatic pressure in sieve tubes at source end of phloem
4. Mass flow to respiring cells or storage tissue
5. Sucrose moved into sink cells by active transport or facilitated diffusion